When routes over a network are not configured redundantly, the buried fiber cut by a roadwork or the power failure in a main switching station due to a natural disaster may disconnect communication. Accordingly, in order to prevent the disconnection of communication, a circuit without a standby route (redundant route) may be added another route that is usable for the same communication for dual routes. In this case, the dual routes in a node-redundant relationship increase the possibility that the disconnection of communication can be prevented even in the event of failure. Here, the expression, “node-redundant relationship”, refers to a relationship in which two routes do not pass through one same node. However, in order to implement the node-redundant routes, the deletion of the original route may sometimes be required.
FIGS. 31A-31E are diagrams illustrating the example in which the deletion of the original route is required when two node-redundant routes are set. It is assumed that, over a network with a network topology as shown in FIG. 31A, a first route having node A as the start node and node Z as the end node is set as shown in FIG. 31B. In this case, while keeping the first route, the redundant route connecting the node A and the node Z may not be set for the first route as the node-redundant route because the redundant route is not allowed passing through both of the node B and the node C. Therefore, in order to set two node-redundant routes, the first route, which is the original route, must be deleted, and the route (second route) indicated by the thick line in FIG. 31C and the route (third route) indicated by the broken line must be set. FIGS. 31D and 31E are diagrams showing the second route and the third route, respectively.
In this way, when the original route must be deleted to set a node-redundant route and when a resource reservation protocol like RSVP (Resource ReSerVation Protocol) is to be used to set a route, a PathTear message and a ResvTear message are used to delete the setting of the resource of the first route. After the deletion of the setting of the first route, the Path message and Resv message are used to secure the second route and third route.
Notably, as a related technology, for example, Japanese Laid-open Patent Publication No. 2002-141943 and R. Braden, Ed., L. Zhang, S. Berson, S. Herzog and S. Jamin, “RFC2205: Resource ReSerVation Protocol (RSVP)—Version 1 Functional Specification”. (URL: http://www.rfc-editor.org/rfc/rfc2205.txt) disclose a method that searches optimal duplicated routes that connect two nodes. In this method, a loop is set over a communication network, and an optimum loop route that connects between transmitting/receiving loops including transmitting/receiving nodes is selected. The node route connecting the transmitting/receiving nodes along the perimeter of the optimum loop route is handled as the shortest duplicated routes.
The above-mentioned route setting method deletes the original route and then sets a redundant route. However, when the original route must be changed to set a redundant route, a part of the links included in the original route is included as a part of the redundant route. In the example shown in FIGS. 31A-31E, in the first route shown in FIG. 31B, the link between the node A and the node B is used by the second route, and the link between the node C and the node Z is used as the third route. Furthermore, because the second route and third route are routes that are used for the same communication as the communication used for the first route, the information to be set as a resource such as a band to be used is basically the same. Therefore, when the first route is deleted and the second route and the third route are then set, unnecessary processing occurs.